Optical lasers are used to provide a modulated light source for optical communication systems. These lasers are designed to have narrow dynamic spectral widths when used in dispersive and/or high data rate systems.
DFB (distributed feedback) lasers are widely used in optical communications systems to produce monomode modulated light output, which is necessary for a narrow linewidth.
Externally modulated lasers operate the laser in a continuous wave (CW) mode, and require the use of expensive external modulators to provide the modulation function. External modulators are typically waveguide-based devices fabricated in lithium niobate or gallium arsenide. The use of external modulation in this way has been required in the past in order to reduce the inherent wavelength chirp of the more simple directly modulated lasers. The cost advantages of using directly modulated lasers have, however, resulted in much research into the use of these devices.
Directly modulated lasers are current-controlled, and the current modulation applied determines the power in the ‘1’ and ‘0’ levels, as well as the output optical frequency. The output is used to define an amplitude modulated signal of a specific frequency without the need for further modulation devices.
There are several sources of non-linearity in a semiconductor laser under modulation.
The basic performance is described by the carrier and photon rate equations given below, which give rise to the damped oscillatory transient response in power and frequency when switching between ‘0’ and ‘1’ levels.
However the presence of non-linearity in the gain mechanism due to intra-band carrier relaxation effects, whereby the gain is not only dependent on the injected carrier density, but also on the photon density or power, gives rise to adiabatic frequency chirp and damping of the laser response.
Additionally the longitudinal non-uniformity in the photon density along the laser cavity gives rise to a longitudinal non-uniformity in carrier density and hence local refractive index, since the presence of injected carriers reduces the refractive index. This, combined with the fixed grating pitch, gives a longitudinal variation in the grating Bragg frequency at which it diffracts light. This effect is known as longitudinal mode spatial hole burning (LMSHB). This means that as the output power is altered, the longitudinal photon, carrier, and refractive index profiles change, and the result is a non-linear variation in the laser output power and frequency with current. Under dynamic conditions, LMSHB can give rise to time constants in the power and frequency response which are related to the carrier lifetime.
Other non-linear effects that can occur include the transverse diffusion of injected carriers from regions of low photon and high carrier density to regions of high photon and low carrier density. Under dynamic conditions the lag associated with the diffusion gives rise to damping.
For timescales of approximately 100 ns-1 μs, the thermal properties of the laser may be important. These are primarily manifested as a frequency chirp associated with the dependence of the refractive index on temperature. If there is a long string of ‘1s’ for example, then the resulting heating in the laser might cause the optical frequency to reduce over a period of ˜100 ns.
Conventional current modulation of a directly modulated semidconductor DFB laser between the “0” and “1” current levels results in a damped oscillatory transient response in power and in frequency. The frequency transient is particularly damaging as it gives rise to a dispersion penalty after propagation through standard optical fibers. It has been recognised that these transient effects, which give rise to dynamic line broadening, can be reduced by shaping the leading edge of the laser current control pulse.
Even when the transient effects are reduced, the other non-linearities associated with directly modulated lasers give rise to difficulties in their use in high speed optical communications systems. Any effect that broadens the laser spectrum also gives rise to distortion after transmission through a dispersive medium. Pure amplitude modulation broadens the spectrum, but any frequency chirp associated with transient effects or other non-linearities, will tend to increase the dispersion-induced distortion further. Chromatic fibre dispersion remains a limiting factor in the development of low cost, high data rate and long reach optical communications systems.
There remains a need for an optical communications system which enables low cost implementation using directly modulated lasers and which provides dispersion compensation as well as taking account of the non-ideal characteristics of the directly modulated optical laser used to modulate the optical carrier.